Despite all the smiles and laughs at the G20 this week in Washington, Christine Lagarde has a message that could not be more serious for those following the U.S. funding debate.

(SOUNDBITE) (English) CHRISTINE LAGARDE, IMF MANAGING DIRECTOR SAYING:

"If there is that degree of disruption, that lack of certainty, that lack of trust in the U.S. signature, it would mean massive disruption the world over, and we would be at risk of tipping yet again into recession, that was the impression around that big table."

Appearing on NBC's "Meet the Press," the managing director of the International Monetary Fund said the U.S. must raise the debt ceiling.

The deadline for the U.S. is Thursday.

If the debt ceiling is not raised, Lagarde warned, then the country could default on its debt, roiling the world financial markets.

(SOUNDBITE) (English) CHRISTINE LAGARDE, IMF MANAGING DIRECTOR SAYING:

"You have to honor your signature, you have to give certainty to the rest of the world and you have to make sure that your own economy is consolidating that welcome economy that we have seen in the last few days, because it impacts the entire economy."

The U.S. government has been shutdown for nearly two weeks as President Obama and House Republicans remain trapped in a fierce budget battle.